Photovoltaic devices provide a non-polluting, solid and reliable source of electrical power. Originally, photovoltaic devices were fabricated from crystalline materials, and as a consequence, were expensive and restricted in size. Techniques have now been developed for the preparation of large area, thin films of semiconductor material which may be advantageously fabricated into low cost, large area, light weight photovoltaic devices.
It is frequently desirable to subdivide large area photovoltaic devices into a plurality of electrically interconnected small area devices which are disposed upon a common substrate. The structure of these arrays makes them more tolerant of defects in the photovoltaic material, and allows for the selection of desired voltage and/or current outputs. In some instances, the small area devices of an array are interconnected in a series arrangement so as to provide for an increased voltage. In other instances, particularly for low voltage, high current applications, a parallel connected array is desired. A parallel connected array provides a constant voltage which is independent of device area. While a single, large area body of photovoltaic material will provide a high current at a low voltage, it is often desirable to configure such large area material into an array of small area devices interconnected in a parallel relationship. Such a parallel connected array allows for localization of defective regions in the photovoltaic material so that such defects may be readily removed without adverse effect on the remainder of the device.
Photovoltaic devices, and particularly thin film photovoltaic devices usually include a transparent, electrically conductive top electrode for collecting photo generated current therefrom. The transparent, electrically conductive electrode material is usually of relatively modest electrical conductivity; therefore, large area devices require current carrying structures such as gridlines, bus bars and the like for decreasing their series resistance. These current carrying structures are fabricated from metals or other such opaque materials, and their presence upon the light incident surface of the photovoltaic device represents a loss of active area. Furthermore, subdividing large area material into a plurality of smaller area devices involves scribing away portions of the semiconductor material, and the scribed regions also represent photovoltaically inactive areas. Clearly, it is desirable to maximize the active area of photovoltaic devices by limiting the size of such inactive areas, while still maintaining low series resistivity in the device.
Various approaches have been implemented in the prior art for manufacture of photovoltaic devices which include a plurality of electrically interconnected small area devices disposed upon a single substrate and having a maximized active area. U.S. Pat. No. 5,268,037 describes a large area photovoltaic array preferably fabricated by a laser scribing process. The array is defined by a plurality of finely scribed lines which subdivide a large area body of photovoltaic material into a series of discrete photovoltaic devices which are electrically connected in parallel and in which current carrying structures are disposed beneath the active semiconductor layer. U.S. Pat. No. 5,468,988 discloses a large area photovoltaic device comprised of a plurality of photovoltaic regions electrically interconnected in parallel. In this device, electrical interconnection is accomplished by means of a large number of relatively small through hole connections which pass through the active semiconductor layer of the device and establish electrical communication between the top and bottom surfaces thereof. The device of the 5,468,988 patent provides a photovoltaic device having a high percentage of photovoltaically active area; however, its manufacture requires that two sets of relatively small through holes be defined in the device at two separate stages of its fabrication, and that these two sets of holes be in a concentric relationship. Vacuum deposition of the top, transparent conductive electrode layer of the device takes place after formation of the second set of holes, which requires introduction and reintroduction of a substrate into a vacuum deposition chamber to complete device fabrication.
It would be desirable to manufacture a large area photovoltaic array by a process which minimizes the number of hole forming steps employed and which also minimizes the number of times a deposition substrate must be placed into, and taken out of a vacuum deposition chamber. In accord with the present invention, and as will be explained in detail hereinbelow, it has been found that in the fabrication of a large area photovoltaic array, conductive paths through the photovoltaic material may be formed by selective heating of the material, and the use of such paths eliminates the step of forming one of the sets of holes thereby eliminating the need for two separate vacuum deposition steps. These and other advantages of the present invention will be apparent from the drawings, discussion and description which follow.